This invention relates to a catalyst for assisting in burning out particulates in engine exhaust, especially exhaust gas from Diesel engines. More particularly, this invention relates to an exhaust purifying catalyst of the type used in combination with a collector for trapping particulates in exhaust gas from an engine, which catalyst allows the temperature required for burning out particulates on the collector to be lowered and assists in burning out a substantial portion of particulates on the collector when the exhaust gas passing through the collector reaches a relatively high temperature under certain operating conditions of the engine.
It was found that particulates in engine exhaust, particularly particulates in exhaust gases from Diesel engines contain carcinogenic substances such as benzpyrene and nitropyrene. It has been a serious problem in view of pollution control to remove such particulates from engine exhaust.
There are known many collectors for collecting and removing particulates from exhaust. Typical are heat resistant collectors such as ceramic honeycombs, ceramic foams, metal mesh fillers, alumina or other pellet-like carrier fillers, and ceramic fibers.
However, as particulates progressively collect and deposit on such collectors, the collectors becomes clogged and do permit smooth passage of exhaust gases. After a certain period of operation, the collectors must be subjected to a burning treatment for regeneration. The burning treatment is generally by burning the particulates on the collectors by means of a burner. The burning treatment is undesirable in that the collectors are deteriorated by a thermal shock caused by the combustion of particulates. There is a likelihood of secondary pollution from noxious gases such as carbon monoxide and nitrogen oxides which have resulted from incomplete combustion.
A concept of carrying catalysts on collectors to lower the burning temperature of collected particulates as well as preventing the generation of noxious gases, was devised.
Based on this concept, the present inventors found several useful catalysts. Some are disclosed in Japanese Patent Application Kokai Nos. 58-18394 and 58-143840. More specifically, Japanese Patent Application Kokai No. 58-18394 discloses an exhaust gas purifying catalyst comprising in combination, (a) at least one member selected from the group consisting of vanadium, molybdenum, and compounds thereof, (b) at least one member selected from the group consisting of alkali metals and compounds thereof, and optionally (c) at least one member selected from the group consisting of copper and compounds thereof.
Japanese Patent Application Kokai No. 58-143840 discloses a particulate purifying catalyst comprising in combination, (a) at least one member selected from the group consisting of copper and copper compounds, for example, copper oxide and (b) at least one member selected from the group consisting of metals capable of taking plural oxidation forms and compounds thereof, for example, germanium, tin, vanadium, niobium, antimony, bismuth, chromium, molybdenum, tungsten, selenium, tellurium, lanthanum, cerium, praseodymium, rhenium, ruthenium, and rhodium, and compounds thereof.
These catalysts, however, are not fully satisfactory with respect to activity and durability as catalysts for assisting in purifying exhaust gases from Diesel engines.
Among the aforementioned catalysts, those catalysts which consist of two components, Cu and V exhibit a very high activity. In order that the catalysts exert their maximum effect, it is desired that the particulate collector having the catalyst carried thereon be located at a position near the engine where a high exhaust gas temperature is available.
The high temperature of exhaust gas gives rise to the problem that sulfur dioxide in the exhaust is oxidized with the aid of catalyst to generate sulfuric acid mist. This problem is serious particularly with trucks and buses which have frequent opportunities to operate under high loads in the course of their normal use and thus discharge exhaust gas at relatively high temperatures.